tests: don't require a D compiler

Reported by Kiyoshi Kanazawa.
http://lists.gnu.org/archive/html/bug-bison/2019-04/msg00018.html

* tests/atlocal.in (BISON_DC_WORKS): New.
* tests/local.at (AT_COMPILE_D): Use it.

tests/atlocal.in

@@ -118,8 +118,14 @@ fi
 ## Other.  ##
 ## ------- ##
 
+# Empty if no D compiler was found.
 : ${DC='@DC@'}
 : ${DCFLAGS='@DCFLAGS@'}
+if test x"$DC" = x; then
+    BISON_DC_WORKS=false
+else
+    BISON_DC_WORKS=true
+fi
 
 # Empty if no javac was found
 : ${CONF_JAVAC='@CONF_JAVAC@'}

tests/local.at

@@ -977,6 +977,7 @@ AT_CHECK(m4_join([ ],
 # with trailing ".o" removed, and ".cc" appended.
 m4_define([AT_COMPILE_D],
 [AT_KEYWORDS(d)
+AT_SKIP_IF([[! $BISON_DC_WORKS]])
 AT_CHECK(m4_join([ ],
                  [$DC $DCFLAGS $3],
                  [m4_bmatch([$1], [[.]], [-c])],